Advise for detecting distance - low power ultra light wei

Discussion in 'General Electronics Chat' started by 4Lester, Jul 26, 2016.

  1. 4Lester

    Thread Starter New Member

    Jul 26, 2016
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    Looking to design a system that can detect an object tethered at 52 feet traveling 100 mph ( on half a hemisphere) and signal when the object moves more than 2 feet inside or outside the path. System will only have a max of 11.1v and weight is critical, no more than 15 grams total system.

    Reading through some of the forums ultrasonic looks like an option but the sensors I am finding commercially are good for -500mm. Can I measure from the tether anchor and just relent from the moving object?

    Hoping the group has some ideas to get me moving in the right direction.
    Thanks -Lester
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to AAC!
    I suspect a laser rangefinder would be about the only type capable of resolving 2 ft at that speed, but am doubtful you'd find one weighing less than 15gm.
    :confused: If it's on a 52ft tether, how can it move 2ft outside the path?
     
  3. 4Lester

    Thread Starter New Member

    Jul 26, 2016
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    Thanks for the note- if/when the tether breaks. We are flying a model airplane sport where two planes flying at the same time are trying to cut the tail off the other plane and sometimes actually cut the other plane away. We are looking to build an engine shut off system so that if the plane gets outside the 52' it triggers and shuts the engine down.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Dead man switch, a loop of wire built into the tether. When the tether breaks, so does the loop. That loss of signal can be turned into engine shutoff with a single transistor. Where does the tether usually break?

    ak
     
  5. 4Lester

    Thread Starter New Member

    Jul 26, 2016
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    ak - we have this type now but it does not detect when the model comes inside the 52' after the pilots and center judge. The tethers break as a result of several issues that come up so never in the same place twice.
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Having flown U-control for some years, are your tethers wire? Is the controller interface (e.g., bellcrank) metal? Can you simply test for a completed electrical circuit?

    If using a single wire, put an audio signal on the wire and check for its absence.

    John
     
  7. 4Lester

    Thread Starter New Member

    Jul 26, 2016
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    Yes we are flying F2D combat and the tethers (2 per plane) are .015" multi strand steal wire. We are actually using low power Rf signal on the lines to a decoding receiver now. We are not having consistent functions due to low power, contections, humidity, and not being able to detect the plane coming inside the circle.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    RF over the wires came several decades after I stopped U-control. Consider that the total movement of the wires relative to each other is relatively small. You might make a stable electrical connection (e.g., crimp or soldered) at the bellcrank and/or handle with a short length of very flexible copper wire (e.g., 24 to 28 awg spaghetti wire).

    Coming inside the circle with intact tethers is a problem. In fact, the one painful incident I had was when my plane did that, and without thinking instantly, I grabbed the wires with my other hand. Dumb, yes, but I was able to crash the plane safely. Next time, I wore gloves.

    Is monitoring tension on the wires out of the question?

    John
     
    Last edited: Jul 26, 2016
  9. 4Lester

    Thread Starter New Member

    Jul 26, 2016
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    Good ideas- we are coming off the flying wire before the bell crank with a soldered wire to a micro connector now, the week piont is he handle to flying wire and flying wire to plane connections.
     
  10. BReeves

    Member

    Nov 24, 2012
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    To add to the problem some guys are now using spider wire fishing line which isn't metallic. Were you involved with the accident at Brodak's where a pilot's arm was cut by the other guys prop when an airplane came into the center of the circle. Easy to imagine an incident like that is reason to research different methods of fuel shutoffs.
     
  11. 4Lester

    Thread Starter New Member

    Jul 26, 2016
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    We are required to run steel cable in our event. Where fishing line is allowed the event runs on US based rules sets - our event is based on international rules set forth by the FAI. I was not in attendance at the Brodak event as it is flown with different rules. Generally the model is safe when it is connected to the cables and operating normally. It is when the variables stack up and the model is cut free or comes inside when it is no longer under the control of the pilot that we are looking for systems to stop the engine.
     
  12. 4Lester

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    Jul 26, 2016
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    Is monitoring tension on the wires out of the question?

    John, I missed this part of your note. How would you monitor the tension?

    Lester
     
  13. jpanhalt

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    Jan 18, 2008
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    Here's a review: http://www.tensionmeters.com/pdf/Tension Sensors.pdf Just search on tension sensor.

    In the three point method, one of the points (usually center) could be spring loaded and actuate a microswitch. Load cells also might be considered. You might also sense when tension is off the bellcrank. I suspect the pilot would need to keep such devices off until the plane was in stable flight.

    This interesting approach was on the first page: http://chemistry.emory.edu/faculty/salaita/Papers_files/ja401494e.pdf One might simply monitor color or refractive index of a link in the control line.

    There is another paper that measures the electrical properties of steel cable. It is possible that some aspect of your cable (e.g., resistance) might change with tension; although, one can imagine that might be hard to standardize for age, temperature, humidity, so forth. Nevertheless an abrupt change might correlate with loss of tension.

    John
     
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